In order to provide batteries having a certain output voltage, it is known to use a plurality of battery cells connected in series. The voltages of the individual battery cells then sum up to the total output of the battery.
In some applications, a rather large number of battery cells are required to achieve the desired output voltage level. For example, in the case of vehicle batteries used in the automotive field, e.g., for electric vehicles or hybrid vehicles, output voltages in the range of 350 V may be needed. For this purpose, about 100 Lithium-ion battery cells, each having a nominal cell voltage of about 3.5 V may be connected in series.
However, the actual cell voltage of a Lithium-ion battery cells may vary considerably depending on the charging state of the battery cell. For example, the cell voltage may be about 4.0 V at full charge and decrease to about 2.5 V at 30% charge. In the above-mentioned example of a battery formed of 100 battery cells, this would correspond to a variation of the output voltage between 400 V and 250 V.
Such voltage variations of a battery may be addressed by appropriate design and dimensioning of other components, e.g., an electric motor of the vehicle or inverter for supplying the electric motor. On one hand, the components need to be capable of handling the maximum output voltage of the battery at full battery charge. On the other hand, the components also need to be capable of handling the increased current flow if the maximum output power is used at low battery charge and thus reduced output voltage of the battery. Such dimensioning and design requirements typically result in increased manufacturing costs. Also, the overall efficiency may be reduced. For example, semiconductor components which have sufficiently high breakthrough voltages for the maximum output voltage of the battery may at the same time have increased losses as compared to semiconductor components with lower breakthrough voltages, which may result in a loss of efficiency.
The problem of varying output voltage may also be addressed by using a DC-DC converter to stabilize the output voltage. However, such a DC-DC converter would need to be dimensioned for the maximum output voltage of the battery, which again may involve considerable costs. Further, usage of a DC-DC converter adds complexity to the battery system and may result in increased costs.
Accordingly there is a need for techniques which allow for efficient battery usage.